1. Field of the Invention
The present invention relates to a liquid crystal projector, and particularly to a liquid crystal projector which uses a plurality of transparent liquid crystal panels and dichroic mirrors to split white light projected from a light source, modulate the light intensity of the split light components and color-light-synthesize the intensity-modulated light components which are then projected onto a screen using a projection lens.
2. Background
A known liquid crystal display (LCD) includes a liquid crystal projector which controls the voltage applied to the liquid crystal to adjust the intensity of light transmitted via the liquid crystal so that a video signal is loaded on the transmitted light. In order to realize a color picture, such a liquid crystal projector comprises a color splitting/synthesizing optical system which splits white light into three primary colors and synthesizes the split light components. As such a color splitting/synthesizing optical system, there have been proposed two kinds of structures, one of which uses two X-shaped dichroic mirrors and the other of which uses four planar dichroic mirrors. The former structure is advantageous in that the optical system is small and the distance between the projection lens and liquid crystal is short to reduce the focal length of the projection lens. However, the structure is disadvantageous in that the X-shaped dichroic mirrors are expensive and a plurality of total reflecting mirrors are required for change the light path, thereby increasing production cost.
Meanwhile, compared with the former structure, in the latter structure using four planar dichroic mirrors, the optical system is relatively large and the distance between the projection lens and liquid crystal is long so as to be disadvantageous in reducing the focal length of the projection lens and the projection distance of light. However, since the latter structure is substantially less expensive that the former structure, it is widely used for the color splitting/synthesizing optical system of liquid crystal projector. A conventional liquid crystal having such color splitting/synthesizing optical system using four planar dichroic mirrors will be described with reference to FIGS. 1A-1E.
FIGS. 1A-1E are graphs of light transmittance characteristics of dichroic mirrors, which show the degree of light transmittance of dichroic mirrors with respect to the spectrum of blue light (B), green light (G) and red light (R). For instance, the dichroic mirror having the light transmittance of FIG. 1A transmits blue light (B) and red light (R) and reflects green light (G).
FIG. 2 illustrates a conventional liquid crystal projector, in which liquid crystal panels (LC panel) and three dichroic mirrors are used to color-split, light-intensity-modulate and synthesize projected light. In FIG. 2, light paths are illustrated by the solid line for blue light (B), the dotted line for green light (G) and the one-dot-one-dash line for red light (R), as indicated. This correlation is also applied in other drawings.
Referring to FIG. 2, the liquid crystal projector includes a light source 11 for generating white light, a color splitting/synthesizing system consisting of four dichroic mirrors 12-15 and a pair of reflecting mirrors 16 and 17, a projection lens 18 for projecting light generated from the color splitting/synthesizing system onto a screen 25, and a fan 29 positioned behind the light source 11. Further, the projector comprises three LC panels 19, 20 and 21 for modulating the light intensity of each of split blue light (B), green light (G) and red light (R), and three condensers 22, 23 and 24 employed for efficient use of the three LC panels 19, 20 and 21. Since light source 11 generates intense light to prevent the picture from darkening due to the loss of light as the light transmits through the LC panels, the fan 29 is installed behind the light source 11 to discharge the heat generated from light source lamp 11 outside the liquid crystal projector.
The dichroic mirrors of FIG. 2 have the following light transmittance characteristics. The first dichroic mirror 12 reflects only the red light (R) of the white light generated from lamp 11; the second and third dichroic mirrors 13 and 14 reflect only blue light (B); and the fourth dichroic mirror 15 reflects only green light (G). In other words, the first dichroic mirror 12 has the light transmittance characteristic of FIG. 1C, the second and third dichroic mirrors 13 and 14 have the light transmittance characteristic of FIG. 1E and the fourth dichroic mirror 15 has the light transmittance characteristic of FIG. 1A.
However, since, due to the characteristics of a coater, coating is performed by applying a large amount of coating material at a time, it is undesirable in regard to production costs to have a large number of dichroic mirrors in the color splitting/synthesizing optical system. Further, with conventional liquid crystal projectors, the dichroic mirror having the light transmittance characteristic of FIG. 1A requires more rigid coating requirements than that required by the other dichroic mirrors having the light transmittance characteristics of FIGS. 1B-1E. Therefore, the presence of the fourth dichroic mirror 15 substantially increases the overall cost of the system.
Further, in the conventional projector, since the light source for generating white light is placed behind the color splitting/synthesizing optical system, the heat produced from the light source lamp is directly transmitted to a viewer and the projector is very large.